Paint comparison examines layer structure and relevant organic and inorganic properties. The sequence, composition and condition of layers can distinguish samples, but similarity alone does not establish one vehicle or object.
Document source areas, transfer and sampling. A database may suggest candidates; it does not replace comparison with the actual suspected source.
By the end of this topic you will be able to:
- Describe paint components and coating-layer sequence.
- Compare microscopy, polymer and elemental methods.
- Evaluate database candidates and physical-fit claims.
- Binder
- The film-forming component, whose composition is assessed with suitable analytical methods.
- Pigment
- A colour or opacity component; elemental composition alone may not identify the compound.
- E-coat
- An electrodeposited coating in some systems, with product-dependent composition and appearance.
- Py-GC-MS
- A thermal-decomposition and separation method requiring validated interpretation, not universal definitive identification.
- PDQ
- Paint Data Query: a paint-system reference resource whose coverage and access must be checked.
- Refinish
- A later coating or repair system; document its actual layers rather than assume one diagnostic structure.
Paint Composition: Binders, Pigments, Solvents and Additives
Binders, pigments, fillers and additives contribute different properties. Curing and weathering change the film; some residual volatiles can persist, so solvent loss is not an absolute rule.
The binder forms the film; pigments contribute colour or opacity; fillers and additives alter properties. A wet coating may contain a carrier that is lost during drying or curing. The examiner therefore considers the actual cured system and its history rather than inferring a complete formulation from colour.
Architectural, Automotive, Industrial
Architectural, automotive and industrial systems vary with product, substrate, repair and age. Avoid universal binder assignments or layer thicknesses.
Automotive Paint Layers and Coating Structure
Record layers from substrate outward, including differences across sampled areas. Repair coatings can complicate a nominal original-equipment sequence.
A cross-section can show the order, number, colour and approximate thickness of layers. Compare questioned and known material in the same orientation, noting missing layers, repairs and weathered surfaces. A matching sequence is more informative than a shared visible topcoat alone, but its frequency and variation still matter.
Sequencing Microscopy and Instrumental Examination
Begin with suitable documented observation and consider a physical fit before destructive work. Sequence microscopy and instrumental methods to preserve information relevant to other examinations.
Begin with examination that documents the sample and preserves possible fracture fit. Microscopy can guide which layers to sample. Subsequent spectroscopy or elemental work should target defined questions, retaining photographs and the link between each analysed portion and its original layer.
Paint Analysis with FTIR, Py-GC-MS and SEM-EDX
FTIR can characterise organic components; pyrolysis-GC-MS can assist polymer comparison; SEM-EDS can assess elemental and morphological features. Each has sampling and interpretation limits.
A peak or element is not a unique manufacturer fingerprint. Compare the complete supported information using appropriate controls.
FTIR compares infrared absorption associated with molecular structure. Pyrolysis-GC-MS examines products formed by controlled thermal decomposition, separation and detection. SEM-EDS combines surface imaging with elemental information. These methods supply complementary observations, rather than interchangeable manufacturer labels.
Collection, the Hit-and-Run Workflow, and PDQ
Collect complete-layer references where feasible and document the actual source area. Separate questioned transfer material and known samples.
Database access and coverage vary. A candidate make, model or production interval is an investigative lead requiring further assessment.
Check Your Understanding
Sources
- 1.Trace evidence analysis · New Jersey State Police
- 2.Trace Evidence Recovery Guidelines (historical technical guidance) · SWGMAT; NIST host
- 3.Forensic Paint Analysis and Comparison Guidelines · SWGMAT; FBI archive